The present invention relates to rotary drilling, and particularly to flow optimization of jet bits during rotary drilling.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 1. In conventional vertical drilling, a drill bit 10 is mounted on the end of a drill string 12 (drill pipe plus drill collars), which may be miles long, while at the surface a rotary drive (not shown) turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, one being the roller cone bit, an example of which is seen in FIG. 2. In this bit a set of cones 16 (two are visible) having teeth or cutting inserts 18 are arranged on rugged bearings such that when rotated about their separate axes, they will effectively cut through various rock formations. The second type of drill bit is a drag bit, having no moving parts, seen in FIG. 3.
Drag bits are becoming increasingly popular for drilling soft and medium formations, but roller cone bits are still very popular, especially for drilling medium and medium-hard rock. There are various types of roller cone bits. One type, insert bits, are normally used for drilling harder formations, and have teeth of tungsten carbide or some other hard material mounted on their cones. As the drill string rotates, the cones roll along the bottom of the hole and the individual hard teeth will induce compressive failure in the formation.
One of the key requirements in drilling is to remove the fractured material from the hole bottom. That is, the formation material to be removed must be 1) fractured, 2) entrained in the flow of drilling fluid, and 3) swept uphole. The function of entraining the fractured material in the flow of drilling fluid is an essential part of this material transport process, especially with roller cone bits. If the fractured material is not efficiently removed from the cutting face, then the impingement of the bit teeth into the intact formation at each pass may be reduced, and energy (and tooth life) may be wasted on further crushing formation material which has already been detached.
Normally a high flow of drilling fluid (typically drilling "mud") is pumped through the drill string to nozzles in the drill bit. Originally, mud was directed at the rotating roller cones, with the purpose of cleaning the cones. With the use of jet bits, in which velocities of hundreds of feet per second are common, the nozzles are normally directed toward the hole bottom. The mud is pumped at high pressures and high flow rates, so that the fluid flow at the hole bottom is very turbulent. The powerful and turbulent flow of mud at the hole bottom helps to separate cuttings from the face, and also cleans the bit and carries away entrained cuttings.
No matter how turbulent the fluid environment is, there will still be a stagnant boundary layer at any solid surface. (This boundary layer becomes thinner at high turbulence, but is always present.) The presence of high-velocity turbulent flows helps to get the cuttings out of the stagnant boundary layer at the hole bottom, but this stage of transport is still an important limit on efficiency.
The roller cone bits which are used in softer formations may have a significant offset angle in their geometries, i.e. the cone axes do not intersect the borehole axis. The geometry of this offset angle makes the teeth scrape across the hole bottom as the bit rotates. (That is, the cones do not roll perfectly, but are always forced to skid at an angle.) This scraping action helps clear cuttings from the hole bottom. Drilling in soft formations tends to produce a much higher volume of cuttings.
During drilling operations, mud is pumped down through the drill string and out nozzles 28 in the drill bit 10, at high pressures and high flow rates. The flow of the mud is one of the most important factors in the operation of the drill bit, serving to remove the cuttings, to cool the drill bit and teeth, and to wash away accumulations of soft material which can clog the bit. Drilling mud also serves to stabilize the borehole and balance the hydrostatic pressure of the formation at the hole bottom. Where these functions are less critical, air, mist, water or other fluids are sometimes used instead of mud.
Background: Nozzles
Within the aperture where mud leaves the bit, removable wear-resistant nozzles determine the size of the opening, and therefore the final velocity of the mud stream. An example can be seen in FIG. 4. In this figure, a nozzle 20 has been inserted into the aperture 14, where it fits snugly. It can be held in place by any one of several means, such as a snap ring 22 (often shrouded to protect the ring from erosion from the mud), screw threads, or a nail lock (where a flexible "nail" is inserted from the edge of the bit to fit into a groove on the outside of the nozzle and inside of the aperture, locking the nozzle in place). At the inside end of the nozzle, its inside diameter is approximately that of the opening above it, while at its outside end, the diameter can be whatever is desired to give the final flow characteristics. To adjust the flow, the nozzle can be replaced with another nozzle which has a different internal diameter at the outside end.
Background: Collimated Flows
In this application, "collimated" refers to fluid streams which contain as little transverse velocity as possible, within the normal constraints of a jet bit. Collimated streams are normally achieved by using a straight nozzle at the end of (and aligned with) a straight passageway which is straight for at least several times the inside diameter of the nozzle. Some drill bit designs use mud flow patterns which are intentionally decollimated. The nozzles eject streams of drilling fluid at high velocities, but they are being injected into a volume of very turbulent flow. Since the velocity of the fluid inside the nozzle is very high, the fluid inside the nozzle is also in a turbulent flow regime. However, it is possible to minimize the lateral components of flow (as is well known), to produce a stream which has as little divergence as possible. This is normally done, in a jet bit, by using a straight nozzle at the end of a straight fluid course which has a central axis aligned with the central axis of the nozzle. FIG. 12 shows a fluid course and nozzle with their axes aligned. The fluid course axis 1204 is identical to the nozzle axis 1202.
Turbulent fluid streams exiting a nozzle diverge as they entrain the surrounding fluid in the hole bottom. The maximum velocity is at the axis of the jet. This axis also defines the stagnation point, the place on the hole bottom where the pressure is at a maximum. The pressure decreases radially from the stagnation point. For detailed analysis of jet impact studies that use this kind of modeling, see, for example, The Effect of Nozzle Diameter on Jet Impact for a Tricone Bit by Warren and Winters, SPE-AIME. FIG. 13 shows how the pressure beneath the nozzle decreases radially from the axis of the nozzle.
Roller Bit with Collimated Jets Sweeping Separate Bottom-Hole Tracks
The present application discloses jet-type roller cone drilling with nozzles which direct collimated streams of mud at different angles, to sweep different radii of the hole bottom. As the bit rotates, each collimated fluid stream attacks a different track along the hole bottom. This provides more efficient removal of cuttings and a higher net rate of penetration.
When the collimated stream exits the nozzle, it immediately begins to diverge. If the cutting face were not close to the nozzle, such a stream would simply mix into the chaotic overall velocity field of the turbulent flow volume below the bit body. The length before the stream dissipates depends on factors such as the nozzle width, the stream velocity, and the relative densities of the stream and of the turbulent flow volume. The present invention teaches that the nozzles should be positioned so that the stream does impact the cutting face. Quantitatively, this can be defined in terms of the peak pressure where the axis of the nozzle points at the cutting face. The present invention teaches that the pressure where the axis of the nozzle intersects the stagnant boundary layer at the cutting face should be at least half of the total pressure inside the nozzle (and ideally equal to or slightly less than the total pressure inside the nozzle).
Thus a collimated stream causes a localized pressure maximum where it hits the cutting face. This localized pressure maximum means that there will be a strong lateral acceleration component at the cutting face (away from this localized pressure maximum, in every direction parallel to the cutting face). This means that the impact of the collimated stream on the cutting face will tend to detach fragments and particles which are partially adhered to the cutting face. Moreover, the high lateral fluid velocities around this local pressure maximum will tend to thin the stagnant boundary layer at the cutting face, so fewer particles can stay entirely within the stagnant boundary layer. Moreover, the high lateral velocities will help to entrain fragments which intersect the overall plane of the boundary layer. Thus this localized pressure maximum is a location of enhanced cuttings removal. The present application teaches that this enhanced cuttings removal effect is best exploited if the localized pressure maxima do not all follow the same track as the bit rotates.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
the use of separate tracks permits highly collimated fluid streams to be used, resulting in better cleaning of the hole bottom; PA1 the use of separate tracks provides more extensive coverage of the hole bottom; and PA1 a higher net rate of penetration results.